<Background Art of First Technique>
In a data processing equipment such as a radio communication equipment or a data storage device, an oscillator whose oscillation frequency is variable is an essential circuit. According to advance of the data processing equipment, an oscillator whose oscillation frequency reaches GHz or so is used recently. When an oscillator is configured using an LC resonant circuit composed of an inductor L and a capacitor C, since values of an inductance or a capacitance become small in such a high frequency, the LC resonant circuit together with a transistor can be easily integrated and formed on a semiconductor substrate. In this case, as the inductor, a narrow line formed in a rectangular shape, a circular shape, or a spiral shape is used, and as the capacitor, a p-n junction capacitor (a diode capacitor), a gate-source/drain capacitor of a MOS (metal oxide semiconductor) transistor (hereinafter, “MOS capacitor”), an MIM (metal insulator metal) capacitor formed between metal layers in a semiconductor device or the like is used. A structure and an operation of the MOS capacitor are disclosed in Sima Dimitrijev, “Understanding Semiconductor Devices,” published by Oxford University Press, pp. 121-129, 2000 (Non-Patent Document 1), for example.
Varying of a frequency is performed by changing a control voltage applied to the diode capacitor or the MOS capacitor or performing switching among a plurality of MIM capacitors, or the like but an inductance is further changed in order to obtain a wide frequency variable range.
Japanese Patent Application Laid-Open No. 2004-15387 (Patent Document 1) discloses a voltage-controlled oscillator as a group of variable capacitance circuit. The voltage-controlled oscillator comprises a parallel circuit of a first variable capacitance circuit including a capacitor element whose capacitance value varies continuously according to a voltage value of a frequency control signal and a second variable capacitance circuit including a capacitor element whose capacitance value varies continuously according to a voltage value of a frequency control signal and being serially connected to a switch circuit. The voltage-controlled oscillator uses an LC resonant circuit that can adjust a variable capacitance by opening and closing the switch circuit controlled according to a capacitor selecting signal. According to Patent Document 1, since a relationship between a fixed capacitance component and a variable capacitance component can be adjusted, fluctuation of a frequency conversion gain (KV) at high and low oscillation frequency ranges of an oscillator can be suppressed.
<Background Art of Second Technique>
In a data processing equipment such as a radio communication equipment or a data storage device, for example, an oscillator whose oscillation frequency is variable is an essential circuit. According to advance of the data processing equipment, an oscillator whose oscillation frequency reaches GHz or so is used recently. When an oscillator is configured using an LC resonant circuit composed of an inductor L and a capacitor C, since values of an inductance or a capacitance become small in such a high frequency, the LC resonant circuit together with a transistor can be easily integrated and formed on a semiconductor substrate. In this case, as the inductor, a narrow line formed in a rectangular shape, a circular shape, or a spiral shape is used, and as the capacitor, a p-n junction capacitor (a diode capacitor), a gate-source/drain capacitor of a MOS (metal oxide semiconductor) transistor (hereinafter, “MOS capacitor”), an MIM (metal insulator metal) capacitor formed between metal layers in a semiconductor device or the like is used.
Varying of a frequency is performed by changing a control voltage applied to the diode capacitor or the MOS capacitor or performing switching among a plurality of MIM capacitors, or the like. In an RF-IC (radio frequency-integrated circuit) for a recent radio communication system, a multi-mode operation meeting two or more kinds of standards or a multi-band operation are required. Since different carrier frequencies are generally used in these standards, a wide frequency variable range is required for an oscillator supplying carriers. For example, an oscillator using a capacitance bank disclosed in Zhenbiao Li and Kenneth K. O, “A low-phase-noise and low-power multiband CMOS voltage-controlled oscillator, “IEEE Journal of Solid-State Circuits, Vol. 40(6), pp. 1296-1302, June, 2005 (Non-Patent Document 2), an oscillator where switching among inductances is performed using a switch that is disclosed in Non-Patent Document 2, or the like is used in order to realize a wide frequency variable range.
Further, not only the wide frequency variable range but also low phase noise characteristic for excellent EVM (error vector magnitude) characteristic or operation at low current due to demand for a low power operation for mobile communication are required for the oscillator used in the RF-IC for a radio communication system. In order to obtain low phase noise characteristic at low current, it is necessary to increase a gain of a negative conductance generating circuit in an oscillator composed of a resonant circuit and the negative conductance generating circuit. For example, a CMOS oscillator disclosed in Nobuyuki Itoh, Shin-ichiro Ishizuka, and Kazuhiro Katoh, “Integrated LC-tuned VCO in BiCMOS process, “Proceedings of the 27th European Solid-State Circuits Conference, 2001, pp. 329-332 (Non-Patent Document 3), a BiPMOS oscillator, or the like is used.